Supernova
If a star starts off with a mass of less that 8 solar masses, then it will stop at the red giant stage. More massive stars continue to burn. The carbon and oxygen produced in the previous stage begin to fuse. Carbon begins to be crushed into neon and magnesium, while oxygen is being crushed into silicon and sulfur. Silicon and sulfur get crushed into an iron core.
This iron core now just sits in the center of the star. The reason for this is that iron doesn't burn. Nuclear burning is only possible if an object is releasing energy. In order for iron to go the fusing process, energy must be added. This leads to the collapse of the star. The star collapses at a rapid rate. In fact it can go from a 5,000 mile diameter to a 12 mile diameter in one second. This tremendous change in size causes the protons and electrons of the contained atoms to combine and form nuetrons. A massive amount of energy equivalent to over 100 stars burning for about 10 billion years. Some energy is released into the shells surrounding the core. This triggers the supernova explosion. The explosion in the inner shells causes a shock wave that blows off the outter layers of the star at over 10 million mph. The extensive amounts of heat and energy produced can allow for the iron in the core fuse into heavier particles. After a a few days the remanents of the star can be as bright as one billion suns, but after a month, the star becomes dimmer and within the next few years will no longer be seen.
After a supernova explosion, the star can become as bright as a 100 million suns, but only for a short amount of time. There are two types of supernovea. The first accurs when the gases from a binary star hit the surface of a white dwarf causing it to explode. The second type of supernovea accurs in stars that are equivalent to 10 solar masses or more. The uncontrolable nuclear reactions in the core cause the star to explode leaving behind nuetron stars and black holes.
(A supernova is different from a nova, in that, the supernova is slightly brighter.)
